1. Field of the Invention
The invention pertains to the field of sonar imaging in marine environment. More particularly, the invention relates to an improved sonar and an associated method of signal processing.
2. Description of Related Art
It is known to “light” the seafloor of which is to be imaged or a submarine target which is to be located with an acoustic pulse, and to listen the time signal generated by the reverberation of the emitted acoustic pulse.
To this end, it is known to use a submarine apparatus, or fish, towed by a surface ship. The flanks of the fish each carry sonars, knows as “lateral sonars”. The acoustic antenna of each of these lateral sonars is formed by a row of several transducers operating in reception, associated with at least one transducer operating in transmission. When submerged to take an image of the seafloor, the fish navigates in such a way to be positioned for example about thirty meters above the seafloor in order to image up to a range of 300 meters. More generally, the fish navigates at a height above the seafloor which corresponds to about 10% of the maximum range of observation. Thus, each lateral sonar makes it possible to observe of a seafloor strip running lengthwise parallel to the moving trajectory of the fish and widthwise in a vertical plane transversal to the trajectory of the fish, within an angular sector originating from the sonar and extending generally between 30° and 80° with respect to the vertical.
Now, the resolution of a sonar, at a given working frequency, substantially depends on the length of the receiving antenna. Thus, to improve the resolution of sonars, the purpose has been, in a first time, to increase the length of their antenna. But the increase of the antenna length is limited by the supporting-fish size. Therefore, it is difficult to make an antenna longer than about 3 m because of the fish bulk aboard the surface ship, and especially because of the difficulties for launching and recovering the fish. Consequently, several means for lengthening the antenna have been proposed.
FR2738918 discloses a synthetic-aperture sonar system which makes it possible, thanks to signal processing device and method, to obtain a virtual antenna greater than the physical antenna, for example about 30 m for the virtual antenna whereas the physical antenna is about 3 m.
However, these device and process imply a limitation of the carrier speed, which must stay lower than half the length of the antenna by recurrence, typically 5 knots for an antenna of 2 m and a range of 300 m.
For example, U.S. Pat. No. 5,747,724 discloses a sonar system intended for seismic studies, having an antenna that is initially folded up inside the body of a submarine towing means and that can be unfolded and towed behind the towing means, which itself can be towed by a surface ship. The antenna consists of a series of acoustic sensors connected to each other by a supporting cable. At the front end of the antenna, a telemetric unit is linked to the towing means by a communication cable.
Such telemetric unit comprises the necessary electronics for multiplexing the signals coming from each of the acoustic sensors and for relaying the multiplexed signal to the towing means, through the communication cable. At the rear end of the antenna, on the side opposite to the towing means, a floating anchor, with a substantial drag, enables the antenna supporting cable to be strained when the towing means moves along, whereby the acoustic sensors are kept in a rigid relative position, essentially rearward, along the trajectory of the towing means.
U.S. Pat. No. 5,747,724 discloses a foldable-antenna sonar that effectively makes it possible to no longer have to use a supporting fish and thus to reduce the bulk of the system, to simplify the launching operations and to be able to construct a great-size antenna having theoretically a very good resolution.
However, when the towing means moves along a trajectory, the described antenna can not be considered as a solid. This antenna has not really a rectilinear shape constant in time, unlike the hypothesis made in U.S. Pat. No. 5,747,724. So, each acoustic sensor is animated with instantaneous movements moving it away from a followed trajectory. These movements result in a degradation of the sonar properties with respect to those expected, and in particular a low resolution, which can be acceptable for applications such as seismic research, in which the working wavelength is of the order of 10 cm or greater, but is not acceptable for smaller wavelengths such as those used for lateral sonars.
One also knows with U.S. Pat. No. 5,117,400 a towed antenna of transducers with an optical calibration which allows to compensate for antenna deviations and U.S. Pat. No. 4,930,111 an overlap correlation processor for a synthetic antenna.
The interest of a deformable-antenna sonar being admitted, there thus exists a need for such an improved deformable-antenna sonar system making it possible to solve the above-mentioned problems, and in particular to effectively obtain high resolutions.